1. Field of the Invention
The present invention relates to a CO2 recovery apparatus and a CO2 recovery method for reducing CO2 in flue gas by allowing CO2 absorbing liquid to absorb CO2 contained in the flue gas, and for regenerating and reusing the CO2 absorbing liquid.
2. Description of the Related Art
It has come to be pointed out that one of the causes of the global warming is a greenhouse effect of CO2, and it has become an urgent task, also internationally, to provide a countermeasure for CO2 to protect the global environment against the warming. CO2 is generated by any human activities combusting fossil fuels, and there are increasing demands for suppressing CO2 emissions. Along with such an increasing demand, researchers are energetically investigating for a method for reducing and recovery CO2 included in flue gas to apply in a power plant that consumes a large amount of fossil fuels, such as a thermal plant. In such a method, flue gas, emitted from a steam generator, is brought into contact with an amine-based CO2 absorbing liquid (hereinafter, also simply referred to as “absorbing liquid”) to allow such absorbing liquid to absorb CO2, and the recovered CO2 is stored therein without being released into the air.
Japanese Application Laid-open No. 2008-62165, for example, discloses a method for allowing an absorbing liquid, such as the one mentioned above, to absorb CO2 in flue gas to reduce the CO2 contained therein, and releasing and recovery the CO2 therefrom. In this method, the absorbing liquid is also regenerated, circulated back to a CO2 absorber, and reused.
An example of a conventional CO2 recovery apparatus is shown in FIG. 4. A conventional CO2 recovery apparatus 100A includes a CO2 absorber 13 and a regenerator 15. The CO2 absorber 13 brings flue gas 11, containing CO2 emitted from industrial combustion facilities such as a steam generator or a gas turbine, into contact with CO2 absorbing liquid 12 to absorb CO2, thus reducing the CO2 contained in the flue gas 11. The regenerator 15 allows CO2 absorbing liquid (hereinafter, also referred to as “rich solvent”) 14 that has absorbed the CO2 to release the CO2 contained therein so that the CO2 absorbing liquid (hereinafter, also referred to as “lean solvent”) 12 can be regenerated.
In FIG. 4, the reference numeral 17 denotes to flue gas having CO2 reduced in the CO2 absorber 13; the reference numeral 18 denotes to a rich solvent pump for pumping the rich solvent 14 into the regenerator 15; the reference numeral 19 denotes to a rich/lean solvent heat exchanger that exchanges heat between the rich solvent 14 and the lean solvent 12; the reference numeral 20 denotes to a lean solvent pump that pumps the regenerated CO2 absorbing liquid 12 into the CO2 absorber 13; the reference numeral 21 denotes to a lean solvent cooler that cools the lean solvent 12; the reference numeral 22 denotes to a regenerating heater; and the reference numeral 23 denotes to steam.
In the CO2 recovery apparatus 100A, the regenerator 15 reduces CO2 in the CO2 absorbing liquid 14 so as to enable the regenerated CO2 absorbing liquid 12 to be reused in the CO2 absorber 13 as CO2 absorbing liquid. CO2 gas 16 removed in the regenerator 15 is compressed in a compressor, injected into underground oilfield, and used for enhanced oil recovery (EOR) or stored in an aquifer as a countermeasure for global warming. The CO2 gas 16 may also be used as synthetic raw material for chemical products.
FIG. 5 is an example of a process of injecting the CO2 gas 16 recovered in the regenerator 15 into underground. The CO2 gas 16 recovered in the regenerator 15 is compressed at a compression process 101, and transported to a well 103a in a storage location by way of transportation 102 such as a pipeline or a ship. At a well 103b at the storage location, the CO2 is mixed with gas (hereinafter, also referred to as “regenerated gas”) 105 generated upon mining crude oil in an accompanying manner, purified in a regenerate gas purifying facility 104, and injected into underground 107 by an injection process 106. At this time, if hydrogen sulfide (H2S) is contained in the regenerated gas 105, oxygen (O2) contained in the CO2 gas 16 may react with the H2S, as expressed in the following formula. By way of such a reaction, solid sulfur (S) may become deposited, and the operation of a plant may be affected:2H2S+O2=2S+2H2O   (1)In addition, if moisture remaining in the CO2 gas 16 is condensed during the compression, the moisture might accelerate carbonic-acid corrosion with co-existence with O2.
In response to this issue, Oil & Gas Journal (issued on Sep. 4, 2006, p 74-84) discloses a method for preventing solid sulfur (S) deposition or carbonic-acid corrosion. In this method, N2 gas and alike is introduced upon starting and stopping a compressor, so that sulfur or O2 remaining in the compressor or a pipe is reduced.
In addition, if the recovered CO2 gas 16 is to be used as a raw material for chemical products, the synthetics may be colored by oxygen. To solve such a problem, it is preferred to reduce oxygen concentration in the recovered CO2 gas 16. The reason why oxygen is contained in the recovered CO2 gas 16 is that oxygen is mixed in the CO2 gas 16 when oxygen contained in the absorbing liquid 12 in the CO2 absorber 13 is released together with CO2 in the regenerator 15.
Japanese Patent Application Laid-open No. 2007-137725, for example, discloses a method for reducing the oxygen concentration in the absorbing liquid. By this method, the oxygen dissolved in the rich solvent 14 is reduced by depressurizing the rich solvent 14 in an oxygen reducing apparatus 24, before pumping the rich solvent 14 into the regenerator 15, as shown in a CO2 recovery apparatus 100B in FIG. 6.
Furthermore, Patent No. 3663117 discloses another method for reducing the oxygen dissolved in the rich solvent. By this method, CO2 gas is used as oxygen-reducing gas, and the CO2 gas is brought in a counter-current contact with the rich solvent, to reduce the oxygen dissolved in the rich solvent.
FIG. 7 is a schematic of a process of compressing the recovered CO2 gas in the regenerator. As shown in FIG. 7, the CO2 gas 16 is released from the top of the regenerator 15, together with the steam released from rich solvent 14 and semi-lean solvent in the regenerator 15, via a gas ejecting line 25. The steam is then condensed in a condenser 26, and water 28 is separated in a separating drum 27. The CO2 gas 16 including the steam is released out of the system, and the pressure of the CO2 gas 16, recovered in the regenerator 15, is gradually raised by way of first compressor 29-1 to fourth compressor 29-4 to compress the CO2 gas 16. The compressed CO2 is then recovered.
First cooler 30-1 to fourth cooler 30-4 and first separator 31-1 to fourth separator 31-4 are respectively arranged downstream of the first compressor 29-1 to the fourth compressor 29-4, respectively, to remove liquid generated while compressing the CO2 gas 16. A dehydrator 33 is arranged between the third compressor 29-3 and the fourth compressor 29-4. In the dehydrator 33, the CO2 gas 16 is brought into contact with dehydrating agent (molecular sieve, diethylene glycol (DEG), or triethylene glycol (TEG), for example) to remove the water and dehydrate the CO2 gas 16.
In FIG. 7, the reference numeral 34 denotes to a gas-liquid separator; and the reference numeral 35 denotes to a condensed-water circulating pump that pumps the water 28, separated in the separating drum 27, to the top of the regenerator 15.
When flue gas containing CO2 is brought into contact with an absorbing liquid in the CO2 absorber, air bubbles can get caught in the absorbing liquid that has flowed down in the CO2 absorber at the bottom thereof, and the rich solvent is sent to the regenerator with the air bubbles being caught. For example, the concentration of oxygen dissolved in the absorbing liquid is approximately several tens of parts per million with respect to the CO2; on the contrary, the concentration of the oxygen getting caught in the absorbing liquid could reach approximately several hundreds of parts per million with respect to the CO2. Therefore, it is necessary to remove the air bubbles getting caught in the rich solvent in the CO2 absorber, to reduce the concentration of oxygen contained in CO2 gas.
As described above, the amount of oxygen getting caught in the absorbing liquid as air bubbles is greater than the amount of oxygen dissolved therein. Because an objective of a conventional method for reducing oxygen is to reduce the oxygen dissolved in the absorbing liquid, motive energy is required in a depressurizing operation or in a gas supply operation to bring purge gas into counter-current contact therewith. Therefore, extra costs are accrued for CO2 recovery.